Dielectric barrier discharge ionization detector

ABSTRACT

A dielectric barrier discharge ionization detector capable of achieving a high signal-to-noise ratio is provided. The detector includes: a discharging section for generating plasma from argon-containing gas by electric discharge; and a charge-collecting section for ionizing a component in a sample gas by an effect of the plasma and for detecting ion current formed by the ionized component. The discharging section includes a cylindrical dielectric tube having a high-voltage electrode connected to AC power source as well as upstream-side and downstream-side ground electrodes and formed on its outer circumferential wall. A semiconductor film is formed on the inner circumferential surface of the tube. The upstream-side and downstream-side ground electrodes are respectively made longer than the initiation distances for a creeping discharge between the high-voltage electrode and a tube-line tip member as well as between the high-voltage electrode and the charge-collecting section.

TECHNICAL FIELD

The present invention relates to a dielectric barrier discharge ionization detector which is primarily suitable as a detector for a gas chromatograph (GC).

BACKGROUND ART

In recent years, dielectric barrier discharge ionization detectors (which are hereinafter abbreviated as the “BIDs”) employing the ionization by dielectric barrier discharge plasma have been put to practical use as a new type of detector for GC (for example, see Patent Literatures 1 and 2 as well as Non Patent Literature 1).

BIDs described in the aforementioned documents are roughly composed of a discharging section and a charge-collecting section which is located below the discharging section. In the discharging section, a low-frequency AC high voltage is applied to plasma-generating electrodes circumferentially formed mound a tube made of a dielectric material, such as quartz glass (“dielectric tube”), to ionize an inert gas supplied into the tube line of the dielectric tube and thereby form atmospheric-pressure non-equilibrium plasma. Due to the effects of the light emitted from this plasma (vacuum ultraviolet light), excited species and other elements, the sample components in a sample gas introduced into the charge-collecting section are ionized. The resulting ions are collected through a collecting electrode to generate detection signals corresponding to the amount of ions, i.e. the amount of sample components.

FIG. 4 shows the configuration of the discharging section and surrounding area in the aforementioned BID. As noted earlier, the discharging section 410 includes a cylindrical dielectric tube 411 made of a dielectric material, such as quartz, the inner space of which forms a passage of inert gas serving as plasma generation gas. On the outer wall surface of the cylindrical dielectric tube 411, three ring-shaped metallic electrodes (made of stainless steel, copper or the like) are circumferentially formed at predetermined intervals of space. A high AC, excitation voltage power source 415 for generating a low-frequency high AC voltage is connected to the central electrode 412 among the three electrodes, while the electrodes 413 and 414 located above and below the central electrode are both grounded. Hereinafter, the central electrode is called the “high-voltage electrode” 412, while the upper and lower electrodes are called the “ground electrodes” 413 and 414. The three electrodes are collectively referred to as the plasma generation electrodes. Since the wall surface of the cylindrical dielectric tube 411 is present between the passage of the inert gas and the plasma generation electrodes, the dielectric wall itself functions as a dielectric coating layer which covers the surface of those electrodes 412, 413 and 414, enabling a dielectric barrier discharge to occur. With the inert gas flowing through the cylindrical dielectric tube 411, when the high AC excitation voltage power source 415 is energized, a low-frequency high AC voltage is applied between the high-voltage electrode 412 and each of the upper and lower ground electrodes 413 and 414 located above and below. Consequently, an electric discharge occurs within the area sandwiched between the two ground electrodes 413 and 414. This electric discharge is induced through the dielectric coating layer (the wall surface of the cylindrical dielectric tube 411), and therefore, is a form of dielectric barrier discharge, whereby the plasma generation gas flowing through the cylindrical dielectric tube 411 is ionized over a wide area, forming a cloud of plasma (atmospheric-pressure non-equilibrium plasma).

The two ground electrodes 413 and 414 arranged so as to sandwich the high-voltage electrode 412 in between prevents the plasma generated by the electric discharge from spreading into the upstream and downstream sections of the cylindrical dielectric tube 411, whereby the effective plasma generation area is confined to the space between the two ground electrodes 413 and 414.

In the BID, the dielectric material which covers the surface of the plasma generation electrodes in the previously described manner prevents an emission of thermions or secondary electrons from the surface of the metallic electrodes. Furthermore, since the plasma generated by the dielectric barrier discharge is a non-equilibrium plasma with low-temperature neutral gas, various factors which cause a fluctuation of the plasma are suppressed, such as a temperature fluctuation in the discharging section or an emission of gas from the inner wall of the quartz tube due to the heat. As a result, the BID can maintain plasma in a stable form and thereby achieve a higher level of signal-to-noise (SN) ratio than the flame ionization detector (FID), which is the most commonly used type of detector for GC.

In general, there are two types of “dielectric barrier discharge”: an electric discharge generated by a configuration in which only one of the high-voltage and ground electrodes is covered with a dielectric body (which is hereinafter called the “single-side barrier discharge”); and an electric discharge generated by a configuration in which both of the high-voltage and ground electrodes are covered with a dielectric body (which is hereinafter called the “double-side barrier discharge”. Non Patent Literature 1 discloses the result of a study in which two discharging sections respectively employing those two configurations were constructed and their detector outputs in a BID-equivalent structure were compared, which demonstrated that a higher SN ratio could be achieved with the double-side barrier discharge than with the single-side barrier discharge.

As the inert gas fir plasma generation in such a BID, helium (He) gas or argon (Ar) gas (or He gas with a trace amount of Ar gas added) is particularly widely used in practice. The reasons for using those gases are as follows:

(1) He gas: The discharge light generated by using He gas has an extremely high energy level of approximately 17.7 eV, making it possible to ionize and detect the atoms and molecules of most substances except for neon (Ne) and He. This is particularly useful for the detection of inorganic substances, since FIDs cannot ionize (and therefore cannot detect) inorganic substances.

(2) Ar gas (or He gas with a trace amount of Ar gas added): The energy level of discharge light generated by using Ar gas is approximately 11.7 eV and cannot ionize inorganic substances, as with the FID. This characteristic is useful in the case of specifically detecting organic substances. For example, in the case of detecting a trace amount of organic substance in an aqueous solution, the trace amount of organic substance of interest can be easily detected since the water used as the solvent cannot be detected.

Since the discharge characteristics vary depending on the kind of gas, the optimum electrode arrangement (e.g. the width of each electrode and the spacing of the electrodes) in the discharging section of the BID also changes depending on whether He gas or Ar gas is used as the inert gas. Accordingly, BIDs are configured to allow users to prepare a plurality of cylindrical dielectric tubes with different electrode arrangements and select a cylindrical dielectric tube having a suitable electrode arrangement for the kind of gas to be used. In the following description, a BID which uses Ar gas (or He gas with a trace amount of Ar gas added) as the plasma generation gas is called the “Ar-BID”. Similarly, a BID which uses He gas as the plasma generation gas is called the “He-BID”.

FIG. 5 is a graph obtained by plotting the discharge initiation voltage far He and Ar at atmospheric pressure against the inter-electrode distance based on Paschen's law which is an empirical law concerning the discharge voltage for spark discharge. As can be seen in the graph, when the inter-electrode distance is the same, the discharge initiation voltage for Ar is approximately two times as high as the discharge initiation voltage for He. In other words, provided that the device should be operated at the same discharge initiation voltage, the inter-electrode distance for Ar needs to be equal to or shorter than one half of the distance for He. Since there are also other parameters affecting the dielectric barrier discharge employed in BIDs, such as the material of the dielectric body, gas purity, frequency of the discharge power source, and waveform of the power source, it is difficult to predict an optimum electrode arrangement and discharging conditions from Paschen's law which is the empirical law concerning, spark discharge. However, from the foregoing discussion, it is at least possible to conclude that the Ar-BID requires a shorter inter-electrode distance between the plasma generation electrodes (or a higher discharge voltage) than the He-BID).

CITATION LIST Patent Literature

Patent Literature 1: JP 2010-60354 A

Patent Literature 2: WO 2012/169419 A

Patent literature 3: JP 2013-125022 A

Non Patent Literature

Non Patent Literature 1: Shinada and four other authors, “Development of New Ionization Detector for Gas Chromatography by Applying Dielectric Barrier Discharge”, Shimadzu Hyouron (Shimadzu Review), Vol. 69, Nos. 3/4, Mar. 29, 2013

SUMMARY OF INVENTION Technical Problem

Due to the previously described reason, the distance between the neighboring electrodes in conventional Ar-BIDs has been made shorter than in He-BIDs. However, an Ar-BID configured in this manner has an evidently lower SN ratio than He-BIDs.

A study to search for the cause of such a decrease in the SN ratio in the Ar-BID has experimentally revealed the fact that, in the case of the electric discharge in Ar gas, although the inter-electrode distance at which the electric discharge can occur is short as noted earlier, once the electric discharge is initiated, the plasma generation area spreads over the entire cylindrical dielectric tube 411 and eventually reaches the tube-line tip member 416 provided at the upper end of the cylindrical dielectric tube 411 as well as the connection member 421 of the charge-collecting section connected to the lower end of the cylindrical dielectric tube 411. Since the tube-line tip member 416 and the connection member 421 are both made of metal and electrically grounded, the electric discharge which occurs within the cylindrical dielectric tube 411 in the aforementioned situation becomes a single-side barrier discharge between the high-voltage electrode 412 which is covered with the dielectric body and the tube-line tip member 416 or connection member 421 which is not covered with any dielectric body. This is the likely reason why the SN ratio was lower than in the case of the double-side barrier discharge.

The present invention has been developed in view of the previously described point. Its objective is to prevent the occurrence of a single-side barrier discharge in the Ar-Bit) and thereby achieve a high SN ratio.

Solution to Problem

The present inventors have inferred that the aforementioned expansion of the plasma generation area beyond the range estimated by Paschen's law occurs due to the creeping discharge occurring at the interface between the inner wall of the cylindrical dielectric tube 411 and the Ar gas. Creeping discharge is a discharge phenomenon which occurs along the boundary surface between different dielectrics. In Ar-BIDS, it is likely that this phenomenon develops from the high-voltage electrode 412 into the upper and lower areas, to eventually induce a gas discharge between the high-voltage electrode 412 and the tube-line tip member 416 as well as between the high-voltage electrode 412 and the charge-collecting section. That is to say, in the aforementioned Ar-BID, since the tube-line tip member 416 or the metallic member (connection member 421) near the upper end of the charge-collecting section is electrically grounded, a potential gradient is formed from the high-voltage electrode 412 toward each of those members 416 and 421. If the ground electrodes 413 and 414 provided between the high-voltage electrode 412 and each of those members 416 and 421 are sufficiently long, the reference potential is spread over a wide range within the space between the high-voltage electrode 412 and each of those members 416 and 421, preventing the development of the creeping discharge. However, in the previously described conventional Ar-BID, since the ground electrodes 413 and 414 are not sufficiently long, the creeping discharge originating from the high-voltage electrode 412 can develop beyond the areas where the ground electrodes 413 and 414 are located, to eventually reach the tube-line tip member 416 or the charge-collecting section, causing the aforementioned expansion of the plasma generation area. Based on such an inference, the present inventors have compared the initiation distance for the creeping discharge in the Ar gas and the same distance in the He gas. The result confirmed that the creeping discharge initiation distance in the Ar gas was longer (i.e. the creeping discharge could occur at a longer distance between the high-voltage electrode 412 and each of the members 416 and 421).

Given this result, the present inventors measured the SN ratio in each case where only one of the ground electrodes 413 and 414 above and below the high-voltage electrode 412 was made longer than the conventional ones. The result demonstrated that the SN ratio particularly improved when the downstream-side ground electrode 414 was made longer. A likely reason for this improvement is that, if the creeping discharge occurs in the downstream area of the flow of the plasma generation gas, i.e. in the area near the charge-collecting section, and causes the plasma generation area to expand into the downstream area, the collecting electrode provided for detecting the ion current in the charge-collecting section suffers from the mixing of electromagnetic noise due to the high voltage or an incidence of charged particles from the plasma.

These facts suggest that, in order to prevent the creeping discharge in sur Ar-BID, it is beneficial to make the ground electrodes, and particularly the one located on the downstream side of the high-voltage electrode, longer than the initiation distance for the creeping discharge in the Ar-BID. However, as shown in FIG. 6, increasing the length of a ground electrode requires increasing the length of the dielectric tube 511 to which the ground electrode 514 is attached, which consequently increases the entire size of the detector. In BIDs, since the charge-collecting section is normally heated to 200° C. or higher temperatures to maintain the sample gas in the gasified state, increasing the detector size leads to an increase in the degree of non-uniformity in the temperature within the dielectric tube, which causes a fluctuation of the output signal.

Thus, a dielectric harrier discharge ionization detector according to one aspect of the resent invention developed for solving the previously described problem is a dielectric barrier discharge ionization detector for ionizing and detecting a sample component in a sample gas by using plasma induced by an electric discharge within a gas passage through which a plasma generation gas containing argon is passed, the detector including:

a) a dielectric tube made of a dielectric material and containing an upstream section of the gas passage;

b) a high-voltage electrode attached to the outer wall of the dielectric tube;

c) a ground electrode electrically connected to a ground and arranged so as to face the gas passage, the ground electrode having a surface which laces the gas passage and is covered with a dielectric body, with at least a portion of the surface being located downstream of the high-voltage electrode in the flow direction of the plasma generation gas;

d) an AC power source connected to the high-voltage electrode, for applying an AC voltage between the high-voltage electrode and the ground electrode so as to induce a dielectric barrier discharge within the gas passage and thereby generate plasma; and

e) a charge-collecting section located downstream of the ground electrode, forming a downstream section of the gas passage, including a sample-gas introducer for introducing a sample gas into the downstream section, and a collecting electrode for collecting ions generated from a sample component in the sample gas by light emitted from the plasma,

where:

a semiconductor film is formed on the inner wall of the dielectric tube over an area where the high-voltage electrode is attached and/or an area located downstream of the aforementioned area and upstream of the charge-collecting section; and

the length of the ground electrode in the area located downstream of the high-voltage electrode is longer than the initiation distance for a creeping discharge between the high-voltage electrode and the charge-collecting section.

The “initiation distance for a creeping discharge between the high-voltage electrode and the charge-collecting section” means the largest length of the ground electrode which allows the creeping discharge to occur between the high-voltage electrode and the charge-collection section under the condition that the ground electrode covered with a dielectric body is located between the high-voltage electrode and the charge-collecting section (the same applies below). For example, in the Ar-BID having the discharging section 410 as shown in FIG. 4 in which the ground electrode 414 is arranged between the high-voltage electrode 412 and the charge-collecting section located downstream of the discharging section 410, if the length of the ground electrode 414 is gradually decreased while the high AC excitation voltage power source 415 is energized, a creeping discharge begins at a certain length, causing a sudden increase in the current value as measured between the high-voltage electrode 412 and the charge-collecting section (e.g. the connection member 421). The length of the ground electrode 414 at which such a sudden increase in the current value occurs is the initiation distance for the creeping discharge.

During the development of the creeping discharge on the surface of a dielectric body, the ionization actively occurs at its front end, forming a highly conductive path behind. However, if a semiconductor film is present on the surface of the dielectric body, the electric charges resulting from the ionization are rapidly diffused through the semiconductor film. Therefore, the aforementioned highly conductive path will not be formed and the creeping discharge will not easily develop. Accordingly, the development of the creeping discharge from the high-voltage electrode toward the charge-collecting section can be impeded by forming a semiconductor film on the inner wall of the dielectric tube over the area where the high-voltage electrode is attached and/or the area located downstream of the aforementioned area and upstream al the charge-collecting section as in the present invention. As a result, the creeping discharge initiation distance within the area between the high-voltage electrode and the charge-collecting section becomes shorter than in the conventional case. Therefore, making the ground electrode longer than the creeping discharge initiation distance does not cause a significant increase in the detector size. That is to say, in the dielectric barrier discharge ionization detector according, to the present invention, it is possible to prevent an occurrence of the creeping discharge while minimizing the increase the detector size.

Although there is no specific limitation on the kind of semiconductor to form the semiconductor film, it is preferable to use a semiconductor which allows for easy formation of the film on the inner wall of the dielectric tube and which also has a low level of reactivity that makes the film hard to be sputtered. Examples of semiconductors having such characteristics include diamond-like carbon and titanium oxide.

The initiation distance for a creeping discharge depends on the kind of semiconductor forming the semiconductor film, the thickness, area and other properties of the semiconductor film as well as such parameters as the frequency and amplitude of the low-frequency AC voltage, waveform of the power source, property of gas (gas purity), and material of the dielectric body. Therefore, “the length of the ground electrode in the area located downstream of the high-voltage electrode” in the present invention should be determined according to those parameters to be applied when the Ar-BID is in use.

The dielectric barrier discharge ionization detector according to the present invention is not limited to a configuration as shown in FIG. 4 in which a high-voltage electrode 412 and two ground electrodes 413 and 414 are circumferentially formed on the outer circumferential surface of a cylindrical dielectric tube 411; it can be applied in various structures of the dielectric barrier discharge ionization detectors. For example, the present invention can also be applied in a dielectric barrier discharge ionization detector disclosed in Patent Literature 3 (a schematic configuration of which is shown in FIG. 7), which includes a high-voltage electrode 612 circumferentially formed on the outer circumferential surface of an external dielectric tube 611 and an electrode structure 634 inserted into the external dielectric tube 611, the electrode structure 634 including an electrically grounded metallic tube 632 (which corresponds to the ground electrode in the present invention) covered with an internal dielectric tube 631 (this example will be detailed later).

When the present invention is applied in the dielectric harrier discharge ionization detector as shown in FIG. 4, the cylindrical dielectric tube 411 corresponds to both the “dielectric tube” and the “dielectric body” covering the ground electrode in the present invention. In other words, the dielectric tube and the dielectric body are formed as a single component. When the present invention is applied in the dielectric harrier discharge ionization detector as shown in FIG. 7, the external dielectric tube 611 corresponds to the “dielectric tube” in the present invention, while the internal dielectric tube 631 corresponds to the “dielectric body” covering the ground electrode in the present invention. In other words, the dielectric tube and the dielectric body are formed as separate components.

A dielectric barrier discharge ionization detector according to another aspect of the present invention is a dielectric barrier discharge ionization detector for ionizing and detecting a sample component in a sample gas by using plasma induced by an electric discharge within a gas passage through which a plasma generation gas containing argon is passed, the detector including:

a) a dielectric tube made of a dielectric material and containing an upstream section of the gas passage;

b) an electrically grounded tube-line tip member made of metal, for introducing the plasma generation gas into the dielectric tube;

c) a high-voltage electrode attached to the outer wall of the dielectric tube;

d) an electrically grounded upstream-side ground electrode attached to the outer wall of the dielectric tube and located upstream of the high-voltage electrode as well as downstream of the tube-line tip member in the flow direction of the plasma generation gas;

e) an electrically grounded downstream-side ground electrode attached to the outer wall of the dielectric tube and located downstream of the high-voltage electrode in the flow direction of the plasma generation gas;

f) an AC power source connected to the high-voltage electrode, for applying an AC voltage between the high-voltage electrode and the upstream-side ground electrode as well as between the high-voltage electrode and the downstream-side ground electrode so as to induce a dielectric barrier discharge within the gas passage and thereby generate plasma; and

g) a charge-collecting section located downstream of the downstream-side ground electrode, for ling a downstream section of the gas passage, including a sample-gas introducer for introducing a sample gas into the downstream section, and a collecting electrode for collecting ions generated from a sample component in the sample gas by light emitted from the plasma,

where:

a semiconductor film is formed on the inner wall of the dielectric tube over an area where the high-voltage electrode is attached and/or an area located downstream of the aforementioned area and upstream of the charge-collecting section; and

the length of the downstream-side ground electrode in the flow direction is longer than the initiation distance for a creeping discharge between the high-voltage electrode and the charge-collecting section.

The above-described configuration is the case where the present invention is applied in a dielectric barrier discharge ionization detector configured as shown in FIG. 4 in which a high-voltage electrode 412 and two ground electrodes 413 and 414 are circumferentially formed on the outer circumferential surface of a cylindrical dielectric tube 411.

As noted earlier, in the dielectric barrier discharge ionization detector, the tube-line tip member 416 provided at the upper end of the discharging section 410 is also made of metal and electrically grounded. Therefore, the creeping discharge originating from the high-voltage electrode 412 can develop not only on the downstream side but also on the upstream side of the same electrode.

Accordingly, the dielectric barrier discharge ionization detector according to the previously described aspect of the present invention may be modified into a dielectric harrier discharge ionization detector for ionizing and detecting a sample component in a sample gas by using plasma induced by an electric discharge within a gas passage through which a plasma generation gas containing argon is passed, the detector including:

a) a dielectric tube made of a dielectric material and containing an upstream section of the gas passage;

b) an electrically grounded tube-line tip member made of metal, for introducing the plasma generation gas into the dielectric tube;

c) a high-voltage electrode attached to the outer wall of the dielectric tube;

d) an electrically grounded upstream-side ground electrode attached to the outer wall of the dielectric tube and located upstream of the high-voltage electrode as well as downstream of the tube-line tip member in the flow direction of the plasma generation gas;

e) an electrically grounded downstream-side ground electrode attached to the outer wall of the dielectric tube and located downstream of the high-voltage electrode in the flow direction of the plasma generation gas;

f) an AC power source connected to the high-voltage electrode, for applying an AC voltage between the high-voltage electrode and the upstream-side ground electrode as well as between the high-voltage electrode and the downstream-side ground electrode so as to induce a dielectric barrier discharge within the gas passage and thereby generate plasma; and

g) a charge-collecting section located downstream of the downstream-side ground electrode, forming a downstream section of the gas passage, including a sample-gas introducer for introducing a sample gas into the downstream section, and a collecting electrode fir collecting ions generated from a sample component in the sample gas by light emitted from the plasma,

where:

a semiconductor film is formed on the inner wall of the dielectric tube over an area where the high-voltage electrode is attached and/or an area located upstream of the aforementioned area as well as downstream of the tube-line tip member; and

the length of the upstream-side ground electrode in the flow direction is longer than the initiation distance tzar a creeping discharge between the high-voltage electrode and the tube-line tip member.

For the purpose of improving the SN ratio, it is preferable make both of the upstream-side and downstream-side ground electrodes longer than the initiation distance for the creeping distance on the upstream and downstream sides of the high-voltage electrode, respectively. However, it causes some problems, such as an increase in the entire length of the dielectric tube, as well as the necessity to supply a considerably high voltage from the AC power source since a configuration for completely preventing the creeping discharge also impedes the intended electric discharge between the high-voltage electrode and each of the upper and lower ground electrodes. Accordingly, the configuration in which only one of the ground electrodes is made longer also has practical merits.

Advantageous Effects of the Invention

As described to this point, the dielectric barrier discharge ionization detector (Ar-BID) according to the present invention configured in the previously described manner can prevent an occurrence of the creeping discharge, whereby the single-side barrier discharge mentioned earlier is prevented and the SN ratio is improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic configuration diagram of an Ar-BID according, to the first embodiment of the present invention.

FIG. 2 shows the electrode arrangement in a test example and a comparative example.

FIG. 3 is a schematic configuration diagram of an Ar-BID according to the second embodiment of the present invention.

FIG. 4 is a schematic configuration diagram of the discharging section and surrounding area in a conventional BID.

FIG. 5 is a graph showing the relationship between the discharge initiation voltage for spark discharge in Ar and He at atmospheric pressure and the inter-electrode distance.

FIG. 6 is a schematic configuration diagram show no a case in which the downstream-side ground electrode of a conventional BID is made longer.

FIG. 7 is a schematic configuration diagram showing still another configuration example of conventional BIDs.

DESCRIPTION OF EMBODIMENTS

Modes for carrying out the present invention are hereinafter described using embodiments.

First Embodiment

FIG. 1 is a schematic configuration diagram of an Ar-BID according to the first embodiment of the present invention.

The Ar-BID of the present embodiment includes a cylindrical dielectric tube 111 made of a dielectric material (e.g. quartz glass) through which a plasma generation gas is passed. In the following description, for convenience of explanation, the vertical direction is defined in such a manner that the upstream side in the flow direction of the gas (indicated by the downward arrows in FIG. 1) in the cylindrical dielectric tube 111 is called the “upper” side, and the downstream side is called the “lower” side. However, this definition does not limit the direction in which the Ar-BID should be used.

On the inner wall surface of the cylindrical dielectric tube 111, a semiconductor film 117 is formed over the entire length of the tube (as will be detailed later). On the outer wall surface al the cylindrical dielectric tube 111, three ring-shaped electrodes made of an electric conductor (e.g. stainless steel or copper) are circumferentially formed at predetermined intervals of space along the flow direction of the gas.

Among the three electrodes, the central electrode 112 has a high AC excitation voltage power source 115 connected, while the two electrodes 113 and 114 located above and below the electrode 112 are both grounded. Hereinafter, the electrodes 112, 113 and 114 are called the “high-voltage electrode”, “upstream-side ground electrode” and “downstream-side ground electrode”, respectively, and these electrodes are collectively called the “plasma generation electrodes”. The high AC excitation voltage power source 115 generates a high AC voltage at a frequency within a range of 1 kHz-100 kHz, more preferably, approximately 5 kHz-30 kHz (low frequency), with an amplitude of approximately 5 kV-10 kV. The AC voltage may have any waveform, such as a sinusoidal, rectangular, triangular or sawtooth wave.

In the Ar-BID of the present embodiment, the area above the lower end of the downstream-side ground electrode 114 in FIG. 1 is the discharging section 110, and the area below the lower end of the downstream-side ground electrode 114 is the charge-collecting section 120.

The cylindrical dielectric tube 111 has a tube-line tip member 116 at its upper end, to which a gas supply tube 116 a is connected. Through this gas supply tube 116 a, a plasma generation gas (Ar gas, or He gas with a trace amount of Ar gas added) doubling as a dilution gas is supplied into the cylindrical dielectric tube 111. Since the wall surface of the cylindrical dielectric tube 111 is present between the plasma generation gas and each of the plasma generation electrodes 112, 113 and 114, the wall surface itself functions as the dielectric coating layer which covers the surfaces of the plasma generation electrodes 112, 113 and 114, enabling dielectric harrier discharge to occur, as will be described later.

On the downstream side of the cylindrical dielectric tube 111, a connection member 121, bias electrode 122 and collecting electrode 123, all of which are cylindrical bodies having the same inner diameter, are arranged along the flow direction of the gas, with insulators 125 a and 125 b made of alumina, PTFE (polytetrafluoroethylene) resin or similar material inserted in between. On the downstream side of the collecting electrode 123, a tube-line end member 124 in the form of a cylindrical body with a closed bottom is attached via an insulator 125 c. The inner space formed by the connection member 121, bias electrode 122, collecting electrode 123, tube-line end member 124 and insulators 125 a, 125 b and 125 c communicates with the inner space of the cylindrical dielectric tube 111.

A bypass exhaust tube 121 a for exhausting a portion of the plasma generation gas to the outside is connected to the circumferential surface of the connection member 121. A sample exhaust tube 124 a is connected to the circumferential surface of the tube-line end member 124. A thin sample introduction tube 126 is inserted through the bottom of the tube-line end member 124. Through this sample introduction tube 126, a sample gas is supplied into the charge-collecting section 120. The charge-collecting section 120 is heated to a maximum temperature of approximately 450° C. by an external heater (not shown) in order to maintain the sample gas in the gasified state.

The connection member 121 is grounded and functions as a recoil electrode for preventing charged particles in the plasma carried by the gas stream from reaching the collecting electrode 123. The bias electrode 122 is connected to a bias DC power source 127. The collecting electrode 123 is connected to a current amplifier 128.

The operation for detecting a sample component contained in a sample gas in the present Ar-BID is hereinafter schematically described. As indicated by the rightward arrow in FIG. 1, a plasma generation gas doubling as a dilution gas is supplied through the gas supply tube 116 a into the cylindrical dielectric tube 111. Since the BID according to the present embodiment is an Ar-BID, either an Ar gas or a He gas containing a trace amount of Ar gas is used as the plasma generation gas. The plasma generation gas flows downward through the cylindrical dielectric tube 111, a portion of which is exhausted through the bypass exhaust tube 121 a to the outside, while the remaining portion serving as the dilution gas flows downward through the charge-collecting section 120, to be exhausted through the sample exhaust tube 124 a to the outside. Meanwhile, the sample as containing the sample component is supplied through the sample introduction tube 126 and ejected from the sample-gas ejection port at the end of the same tube into the charge-collecting section 120. Although the direction in which the sample gas is ejected from the sample-gas ejection port is opposite to the flow direction of the dilution gas, the sample gas is immediately pushed backward, being merged with the dilution gas and flowing downward, as indicated by the arrows in FIG. 1.

As noted earlier, while the plasma generation gas is flowing through the cylindrical dielectric tube 111, the high AC excitation voltage power source 115 applies a high AC voltage between the high-voltage electrode 112 and the upstream-side ground electrode 113 as well as between the high-voltage electrode 112 and the downstream-side ground electrode 114. As a result, a dielectric barrier discharge occurs within the cylindrical dielectric tube 111, whereby the plasma generation gas is ionized and a cloud of plasma (atmospheric pressure non-equilibrium plasma) is generated. The excitation light emitted from the atmospheric-pressure non-equilibrium plasma travels through the discharging section 110 and the charge-collecting section 120 to the region where the sample gas is present, and ionizes the sample component in the sample gas. The thereby generated ions move toward the collecting electrode 123 due to the effect of the electric field created by the DC voltage applied to the bias electrode 122. Upon reaching the collecting electrode 123, the ions give electrons to or receive electrons from the same electrode. Consequently, an ion current corresponding to the amount of ions generated from the sample component, i.e. an ion current corresponding to the amount of sample component, is fed to the current amplifier 128. The current amplifier 128 amplifies this current and produces a detection signal. In this manner, the Ar-BID according to the present embodiment produces a detection signal corresponding to the amount (concentration) of the sample component contained in the sample gas introduced through the sample introduction tube 126.

The basic components of the BID in the present embodiment are the same as those of commonly used BIDs. The previously described basic operation for detection is also similar to that of commonly used BIDs. The structural characteristics of the Ar-BID according to the present embodiment exist in that the inner wall surface of the cylindrical dielectric tube 111 is covered with a semiconductor film 117, and that the upstream-side ground electrode 113 and the downstream-side ground electrode 114 are respectively made longer than the creeping discharge initiation distances between the high-voltage electrode 112 and the tube-line tip member 116 as well as between the high-voltage electrode 112 and the charge-collecting section 120 (more specifically, the connection member 121).

When the semiconductor film 117 is formed on the inner wall surface of the cylindrical dielectric tube 111 in the previously described manner, the creeping discharge initiation distances between the high-voltage electrode 112 and the tribe-line tip member 116 as well as between the high-voltage electrode 112 and the connection member 121 become shorter than when no semiconductor film 117 is formed. As a result, the length of the upstream-side ground electrode 113 and that of the downstream-side ground electrode 114 necessary for preventing the creeping discharge also become shorter than when no semiconductor film 117 is formed (e.g. than the electrode 514 in FIG. 6). Accordingly, in the Ar-BID according to the present embodiment, it is possible to prevent an occurrence of the creeping discharge and thereby improve the SN ratio while minimizing the increase in the detector size.

Although the semiconductor film 117 may be any kind of material, it is preferable to use a semiconductor which has a low level of reactivity that makes the film resistant to sputtering and which also allows for easy formation of the film on the inner wall of the cylindrical dielectric tube 111. For example, a diamond-like carbon (DLC) film which cast be formed by plasma chemical vapor deposition (CVD) or a titanium oxide (TiO₂) film which can be formed by a sol-gel process can be suitably used as the semiconductor film.

In the present embodiment, the length of the downstream-side ground electrode 114 is adjusted according to the kind and thickness of the semiconductor film as well as such parameters as the frequency and amplitude of the low-frequency AC voltage, waveform of the power source, and property of gas so that no creeping discharge will occur between the high-voltage electrode 112 and the charge-collecting section 120. The length of the upstream-side ground electrode 113 is also adjusted according to the kind and thickness of the semiconductor film as well as the aforementioned parameters so that no creeping discharge will between the high-voltage electrode 112 and the tube-line tip member 116.

In the previously described example, the semiconductor film 117 is formed over the entire length of the cylindrical dielectric tube 111. However, the minimum requirement is to form the semiconductor film 117 on at least one of the following portions of the cylindrical dielectric tube 111: the inner circumferential surface of the area which covers the high-voltage electrode 112 (this area is hereinafter called the “high-voltage electrode coverage area”); the inner circumferential surface of the area located downstream of the high-voltage electrode coverage area as well as upstream of the charge-collecting section 120 (this area is hereinafter called the “downstream-side area”); and the inner circumferential surface of the area located upstream of the high-voltage electrode coverage area as well as downstream of the tube-line tip member 116 (this area is hereinafter called the “upstream-side area”). Providing, the semiconductor film on the high-voltage electrode coverage area and/or the downstream-side area shortens the creeping discharge initiation distance between the high-voltage electrode 112 and the charge-collecting section 120. Providing the semiconductor film on the high-voltage electrode coverage area and/or the upstream-side area shortens the creeping discharge initiation distance between the high-voltage electrode 112 and the tube-line tip member 116.

In the previously described example, both the upstream-side ground electrode 113 and the downstream-side ground electrode 114 are made longer than the respective creeping discharge initiation distances. It is also possible to make only one of them, and particularly, only the downstream-side ground electrode 114, longer than the creeping discharge initiation distance (between the high-voltage electrode 112 and the charge-collecting section 120). In the case of making only the downstream-side ground electrode 114 longer than the creeping discharge initiation distance, the semiconductor film is formed on the inner circumferential surface of at least either be high-voltage electrode coverage area or the downstream-side area. In the case of making only the upstream-side ground electrode 113 longer than the creeping discharge initiation distance, the semiconductor film is formed on the inner circumferential surface of at least either the high-voltage electrode coverage area or the upstream-side area.

Test Example

Hereinafter described is a test conducted for confirming the effect of the Ar-BID according to the first embodiment. The test was performed using an Ar-BID in which a semiconductor film made of DLC was added to a portion of the inner circumferential surface of the cylindrical dielectric tube (this BID is hereinafter called the “test example”) as well as an Ar-BID including the cylindrical dielectric tube with no semiconductor film added (this BID is hereinafter called the “comparative example”). FIG. 2 shows the electrode arrangement in the discharging section of the Ar-BIDS in the test example and the comparative example. It should be noted that the semiconductor film 217 was formed only in the test example, at the shown location. In both of the test and comparative examples, the cylindrical dielectric tube 211 was a quartz tube measuring 4 mm in outer diameter, 2 mm in inner diameter and 92 mm in length. Strips of copper toil were wound on the outer circumferential surface of the cylindrical dielectric tube 211 to thrill the high-voltage electrode 212, upstream-side ground electrode 213 and downstream-side ground electrode 214. The electrode arrangement in FIG. 2 was determined so that a creeping discharge would occur on the upstream side of the high-voltage electrode 212 while no creeping discharge would occur on the downstream side when the measurement was performed using the cylindrical dielectric tube 211 with no semiconductor film under measurement conditions which will be described later. In other words, the upstream-side ground electrode 213 was shorter than the initiation distance for the creeping discharge between the high-voltage electrode 212 and the tube-line tip member 216 under the aforementioned conditions in the Ar-BID of the comparative example, while the downstream-side ground electrode 214 was longer than the initiation distance for the creeping discharge between the high-voltage electrode 212 and the connection member 221 under the same conditions in the Ar-BID of the comparative example.

Using each of those Ar-BIDs as the detector for GC, the sensitivity for a solution of a standard sample (dodecane) was measured, with Ar gas (with a degree of purity of 99.9999% or higher) continuously introduced into the cylindrical dielectric tube 211, and the high AC excitation voltage power source 215 energized to apply an AC high voltage having a sinusoidal current waveform at a frequency of approximately 40 kHz with a voltage amplitude of approximately 5 kVp-p. The detection limit was also calculated for each case from the measured noise value. Table 1 below shows the measured results and the calculated results based on the measured results.

TABLE 1 Sensitivity Noise Detection Limit (C/g) (fA) (pg/sec) Test Example 0.78 90 0.23 Comparative Example 1.0 135 0.27

As shown in Table 1, the sensitivity in the test example was lower than in the comparative example. This seems to be due to the fact that the shortened initiation distance for the creeping discharge as compared to the comparative example suppresses not only the creeping discharge developing from the high-voltage electrode 212 into the downstream area but also the creeping discharge developing into the upstream area, causing the discharge area on the upstream side of the high-voltage electrode 212 to shrink and the amount of discharge light to decrease. However, in the test example, not only the sensitivity but also the noise value was decreased, with the result that the detection limit was higher than in the comparative example. The SN ratio was also higher than in the comparative example. The most likely explanation for this improvement in the SN ratio is that the shrinkage of the discharge area on the upstream side of the high-voltage electrode 212 enabled the electric discharge on the upstream side, which occurs as the single-side barrier discharge in the comparative example, to be a double-side barrier discharge in the test example.

As just described, in the test example, the SN ratio could be improved even on the upstream area where the ground electrode 213 had a relatively short length. Therefore, in the Ar-BID of the test example, it should be possible to make the downstream-side ground electrode 214 as short as the upstream-side ground electrode 213 and yet prevent the creeping discharge from developing from the high-voltage electrode 212 into the downstream area, by extending the area of the semiconductor film 217 (DLC film) on the inner wall of the cylindrical dielectric tube 211 into the area located downstream of the high-voltage electrode 212.

As noted earlier, the initiation distance for a creeping discharge depends on such parameters as the kind, thickness and area of the semiconductor film as well as the frequency and amplitude of the low-frequency AC voltage, waveform of the power source, property of gas, and material of the dielectric body. Accordingly, the lengths of the ground electrodes in the Ar-BID) according to the present embodiment are not limited to the values shown in FIG. 2, but should be appropriately determined according to the configuration and use conditions of the Ar-BID. For example, under the condition that the various aforementioned parameters are fixed, the length of the downstream-side or upstream-side ground electrode may be variously changed to locate a point at which a sudden change occurs in a specific quantity, such as the size of the plasma generation area, amount of the current flowing between the high-voltage electrode (112 in FIG. 1) and the connection member (121 in FIG. 1), amount of the current flowing between the high-voltage electrode and the tube-line tip member (116 in FIG. 1), or SN ratio during a sample measurement. The length of the ground electrode at the located point can be considered to correspond to the initiation distance for the creeping discharge. Therefore, by making the actual length of the downstream-side or upstream-side ground electrode larger than the length corresponding to the aforementioned point, the creeping discharge can be suppressed and a high SN ratio can be achieved. Furthermore, if the length of the ground electrode at which the creeping discharge ceases is previously investigated with various values of the aforementioned parameters, it becomes possible to estimate the length of the ground electrode with which a high SN ratio can be achieved under a given condition.

Second Embodiment

The second embodiment of the Ar-BID according to the present invention is hereinafter described with reference to FIG. 3. FIG. 3 is a schematic configuration diagram of the Ar-BID according to the present embodiment.

The Ar-BID of the present embodiment includes an external dielectric tube 311 made of a dielectric material, such as quartz. For example, a quartz tube measuring 7 mm in outer diameter and 5 mm in inner diameter can be used as the external dielectric tube 311. A semiconductor film 317 is formed over the entire length of the inner wall surface of the external dielectric tube 311 (as will be detailed, later). A ring-shaped electrode 312 made of metal (e.g. stainless steel or copper) is circumferentially formed on the outer circumferential surface of the external dielectric tube 311.

At the upper end of the external dielectric tube 311, a tube-line tip member 316 having a cylindrical shape with a closed top and an open bottom is attached. A gas supply tube 316 a is connected to the circumferential surface of the tube-line tip member 316. The tube-line tip member 316 and the gas supply tube 316 a are made of metal, such as stainless steel.

Inside the external dielectric tube 311, an internal dielectric tube 331 made of a dielectric material (e.g. quartz) is arranged, and a metallic tube 332 made of metal (e.g. stainless steel) is inserted into this internal dielectric tube 331. Furthermore, an insulating tube 333 made of alumina or the like is inserted into the metallic tube 332, and a metallic wire 322 made of metal (e.g. stainless steel) is inserted into the insulating tube 333. The internal dielectric tube 331, metallic tube 332, insulating tube and metallic wire 322 have their respective lengths sequentially increased in the mentioned order, with the upper and lower ends of the metallic tube 332 protruding from the upper and lower ends of the internal dielectric tube 331, and the upper and lower ends of the insulating tube 333 protruding from the upper and lower ends of the metallic tube 332. The upper and lower ends of the metallic wire 322 also protrude from the upper and lower ends of the insulating tube 333. The structure composed a the internal dielectric tube 331, metallic tube 332, insulating tube 333 and metallic wire 322 is hereinafter called the “electrode structure 334”.

The tube-line tip member 316 has a through hole formed in its upper portion. The upper end of the metallic tube 332 is fixed in this through hole by welding or soldering. The insulating tube 333 and the metallic wire 322 are extracted to the outside from the through hole in the upper portion of the tube-line tip member 316, and sealed and fixed on the upper surface of the tube-line tip member 316 with a gas-tight adhesive 316 b.

The tube-line up member 316 is electrically grounded through an electric line (or gas supply tube 316 a), whereby the metallic tube 332 is also electrically grounded via the tube-line tip member 316. On the other hand, the ring-shaped electrode 312 has a high AC excitation voltage power source 315 connected to it. That is to say, in the Ar-BID of the present embodiment, the ring-shaped electrode 312 corresponds to the high-voltage electrode in the present invention, the area of the metallic tube 332 covered with the internal dielectric tube 331 (this area is hereinafter called the “dielectric coverage area”) corresponds to the ground electrode in the present invention, and the ring-shaped electrode (high-voltage electrode) 312 and the dielectric coverage area of the metallic tube 332 (ground electrode) function as the plasma generation electrodes. The inner circumferential surface of the ring-shaped electrode 312 faces a portion of the outer circumferential surface of the metallic tube 332 across the wall surfaces of the external dielectric tube 311 and the internal dielectric tube 331. Accordingly, those dielectric wall surfaces themselves function as dielectric coating layers which cover the surfaces of the plasma generation electrodes (i.e. ring-shaped electrode 312 and metallic tube 332), enabling a dielectric barrier discharge to occur.

In the present embodiment, the area above the lower end of the internal dielectric tube 331 in FIG. 3 corresponds to the discharging section 310, and the area below the lower end of the internal dielectric tube 331 corresponds to the charge-collecting section 320.

The lower end of the external dielectric tube 311 is inserted into the cylindrical connection member 321. A bypass exhaust tube 321 a made of metal (e.g. stainless steel) is provided on the circumferential surface of the connection member 321.

Below the connection member 321, there are a cylindrical insulating member 325 a, flanged metallic tube 323, cylindrical insulating member 325 b, and tube-line end member 324 arranged in the mentioned order. The flanged metallic tube 323 has a cylindrical portion 323 a and a flange portion 323 b which is formed at the lower end of the cylindrical portion 123 a and extends outward in the radial direction of the cylindrical portion 323 a. The cylindrical portion 323 a has an outer diameter smaller than the inner diameter of the external dielectric tube 311 and is inserted in the external dielectric tube 311 from below. The flange portion 323 b, which has approximately the same outer diameter as those of the connection member 321, insulating members 325 a, 325 b and tube-line end member 324, is held between the lower end of the connection member 321 and the upper end of the tube-line end member 324 via the insulating members 325 a and 325 b. The connection member 321, tube-line end member 324 and flanged metallic tube 323 are all made of metal (e.g. stainless steel). The connection member 321, insulating member 325 a, flanged metallic tube 323, insulating member 325 b, and tube-line end member 324 are each adhered to the neighboring members with a heat-resistant ceramic adhesive.

The tube-line end member 324 is a cylindrical member having an open top and a closed bottom, with a sample exhaust tube 324 a made of metal (e.g. stainless steel) connected to its circumferential surface. A through hole is formed in the bottom surface of the tube line end member 324, and a sample introduction tube 326 connected to the exit end of a GC column (or similar element) is inserted into the through hole. The sample introduction tube 326 is pulled into the cylindrical portion 323 a of the flanged metallic tube 323. The upper end (i.e. sample-gas exit port) of the sample introduction tube 326 is located at a vertical position between the upper and lower ends of the cylindrical portion 323 a.

As described earlier, a portion which is not covered with the insulating tube 333 (“exposed portion”) is provided at the lower end of the metallic wire 322 in the electrode structure 334. The exposed portion is inserted into the cylindrical portion 323 a of the flanged metallic tube 323 from above and is located near the upper end of the cylindrical portion 323 a. As a result, the exposed portion of the metallic wire 322 is located directly above the sample-gas exit port. Furthermore, the metallic wire 322 is extracted from the tube-line tip member 316 to the outside and connected to a bias DC power source 327. The flanged metallic tube 323 is connected to a current amplifier 328. That is to say, in the Ar-BID of the present embodiment, the exposed portion at the lower end of the metallic wire 322 functions as the bias electrode, while the cylindrical portion 323 a of the flanged metallic tube 323 functions as the ion-collecting electrode. Accordingly, the space between the inner wall of the cylindrical portion 323 a and the exposed portion of the metallic wire 322 is the effective ion-collecting area.

As noted earlier, the metallic tube 332 included in the electrode structure 334 is grounded via the tube-line tip member 316, and a portion which is not covered with the internal dielectric tube 331 (“exposed portion”) is provided at the lower end of the metallic tube 332. This exposed portion is located directly above the flanged metallic tube 323 and functions as a recoil electrode for preventing charged particles in the plasma from reaching the ion-collecting electrode (i.e. the cylindrical portion 323 a).

A detecting operation by the present Ar-BID is hereinafter described. As indicated by the rightward arrow in FIG. 3, a plasma generation gas (Ar gas, or He gas containing a trace amount of Ar gas) doubling as a dilution gas is supplied through the gas supply tube 316 a into the tube-line tip member 316.

The plasma generation gas doubling as the dilution gas flows downward through the space between the inner wall of the external dielectric tube 311 and the outer wall of the internal dielectric tube 331. At the upper end of the cylindrical portion 323 a of the flanged metallic tube 323, a portion of the gas is made to branch off. The branch portion of the plasma generation gas flows downward through the space between the inner wall of the external dielectric tube 311 and the outer wall of the cylindrical portion 323 a. At the lower end of the external dielectric tube 311, the flow turns outward, and then upward. After flowing upward through the space between the outer wall of the external dielectric tube 311 and the inner wall of the connection member 321, the gas is exhausted through the bypass exhaust tube 321 a to the outside. Meanwhile, the remaining portion of the plasma generation gas flows into the space surrounded by the inner wall of the cylindrical portion 323 a, as the dilution gas to be mixed with the sample gas.

While the plasma generation gas is flowing through the space between the inner wall of the external dielectric tube 311 and the outer wall of the internal dielectric tube 331 in the previously described manner, the high AC excitation voltage power source 315 is energized. The high AC excitation voltage power source 315 applies a low-frequency high AC voltage between the plasma generation electrodes, i.e. the ring-shaped electrode (high-voltage electrode) 312 and the dielectric coverage area (ground electrode) of the metallic tube 332. Consequently, an electric discharge occurs within the area sandwiched between the ring-shaped electrode 312 and the dielectric coverage area of the metallic tube 332. This electric discharge is induced through the dielectric coating layers (external dielectric tube 311 and internal dielectric tube 331), and therefore, is a dielectric barrier discharge. By this dielectric barrier discharge, the plasma generation gas flowing through the space between the inner wall of the external dielectric tube 311 and the outer wall of the internal dielectric tube 331 is ionized, forming a cloud of plasma (atmospheric-pressure non-equilibrium plasma).

The excitation light emitted from the atmospheric-pressure non-equilibrium plasma travels through the space between the inner wall of the external dielectric tube 311 and the outer wall of the internal dielectric tube 331 to the region where the sample gas is present, and ionizes the molecules (or atoms) of the sample component in the sample gas. The thereby generated sample ions are gathered to the ion-collecting electrode (i.e. the cylindrical portion 323 a of the flanged metallic tube 323) due to the electric field created by the bias electrode (i.e. the exposed portion of the metallic wire 322) located directly above the sample-gas exit port, to be eventually detected as a current output. Consequently, an ion current corresponding to the amount of generated sample ions, i.e. an ion current corresponding to the amount of sample component, is fed to the current amplifier 328. The current amplifier 328 amplifies this current and provides it a detection signal. In this manner, the present Ar-BID produces a detection signal corresponding to the amount (concentration) of the sample component contained in the introduced sample gas.

FIG. 3, the metallic wire 322 is made to function as the bias electrode, and the flanged metallic tube 323 is made to function as the ion-collecting electrode. Their functions may be transposed. That is to say, the metallic wire 322 may be connected to the current amplifier 328, and the flanged metallic tube 323 may be connected to the bias DC power source 327. It is also possible to replace the flanged metallic tube 323 with an element similar to the cylindrical metallic electrode 122 or 123 provided in the charge-collecting section in FIG. 1, and, make this element function as the ion-collecting electrode or bias electrode.

The basic components and detecting operation of the Ar-BID in the present embodiment are the same as those of the BID described in Patent Literature 3. FIG. 7 shows the configuration of the BID described in Patent Literature 3. In FIG. 7, the components which are common to FIGS. 3 and 7 are denoted by numerals whose last two digits are common to both figures.

The structural characteristics of the Ar-BID of the present embodiment exist in the following points: the inner wall surface of the external dielectric tube 311 is covered with the semiconductor film 317; the length of the dielectric coverage area of the metallic tube (ground electrode) 332 on the downstream side of the lower end of the ring-shaped electrode (high-voltage electrode) 312 is made longer than the creeping discharge initiation distance between the ring-shaped electrode 312 and the charge-collecting section 320; and the length of the dielectric coverage area of the metallic tube (ground electrode) 332 on the upstream side of the upper end of the ring-shaped electrode (high-voltage electrode) 312 is made longer than the creeping discharge initiation distance between the ring-shaped electrode 312 and the tube-line tip member 316.

When the semiconductor film 317 is formed on the inner wall surface of the external dielectric tube 311 in the previously described manner, the creeping discharge initiation distance between the high-voltage electrode 312 and the tube-line tip member 316 as well as between the high-voltage electrode 312 and the charge-collecting section (e.g. the lower-end area of the metallic tube 332 which is not covered with the internal dielectric, tube 331, or the upper-end portion of the flanged metallic tube 323) becomes shorter than when no semiconductor film 317 is formed. As a result, the length of the dielectric coverage area of the metallic tube 332 necessary for preventing the creeping discharge also become shorter (than when no semiconductor film 317 formed). Accordingly, in the Ar-BID according to the present embodiment, it is possible to prevent an occurrence of the creeping discharge and thereby improve the SN ratio while minimizing the increase in the detector size.

Although the semiconductor film 317 may be any kind of material, it is preferable to use a semiconductor which has a low level of reactivity that makes the film resistant to sputtering and which allows for easy formation of the film on the inner circumferential surface of the external dielectric tube 311. For example, a diamond-like carbon (DLC) film which can be formed by plasma chemical vapor deposition (CVD) or a titanium oxide (TiO₂) film which can be formed by a sol-gel process can be suitable used as the semiconductor film.

In the previously described example, the semiconductor film 317 is formed over the entire length of the external dielectric tube 311. However, the minimum requirement is to form the semiconductor film 317 on a least one of the following portions of the external dielectric tube 311: the inner circumferential surface of the area which covers the ring-shaped electrode 312 (this area is hereinafter called the “ring-shaped electrode coverage area”); the inner circumferential surface of the area located downstream of the ring-shaped electrode coverage area as well as upstream of the charge-collecting section 320 (this area is hereinafter called the “downstream-side area”); and the inner circumferential surface of the area located upstream of the high-voltage electrode coverage area as well as downstream of the tube-line tip member 316 (this area is hereinafter called the “upstream-side area”). Providing the semiconductor film 317 on the high-voltage electrode coverage area and/or the downstream-side area shortens the creeping discharge initiation distance between the high-voltage electrode 312 and the charge-collecting section 320. Providing the semiconductor film 317 on the ring-shaped electrode coverage area and/or the upstream-side area shortens the creeping discharge initiation distance between the high-voltage electrode 312 and the tube-line tip member 316.

In the previously described example, the dielectric coverage area of the metallic tube 332 is made longer than the creeping discharge initiation distance on both the upstream side of the upper end of the ring-shaped electrode 312 and the downstream side of the lower end of the ring-shaped electrode 312. It is also possible to make the dielectric coverage area of the metallic tube 332 on only one of the two sides, and particularly, only on the downstream side of the lower end of the ring-shaped electrode, longer than the creeping discharge initiation distance (between the ring-shaped electrode 312 and the charge-collecting section 320). In the ease of making only the dielectric coverage area of the metallic tube 332 on the downstream side of the lower end of the ring-shaped electrode 312 longer than the creeping discharge initiation distance, the semiconductor film 317 is formed on the inner circumferential surface of at least either the ring-shaped electrode coverage area or the downstream-side area. In the case of making only the dielectric coverage area of the metallic tube 332 on the upstream side of the upper end of the ring-shaped electrode 312 longer than the creeping discharge initiation distance, the semiconductor film 317 is formed on the inner circumferential surface of at least either the ring-shaped electrode coverage area or the upstream-side area.

The length of the dielectric coverage area of the metallic tube 332 on the downstream side of the lower end of the ring-shaped electrode 312 is adjusted according to the kind, thickness and area of the semiconductor film 317 on the ring-shaped electrode coverage area and the downstream-side area of the external dielectric tube 311 as well as such parameters as the frequency and amplitude of the low-frequency AC voltage, waveform of the power source, material of the dielectric body, and property of gas so that no creeping discharge will occur between the ring-shaped electrode 312 and the charge-collecting section 320. The length of the dielectric coverage area on the upstream side of the upper end of the ring-shaped electrode 312 is adjusted according to the kind, thickness and area of the semiconductor film 317 on the ring-shaped electrode coverage area and the upstream-side area as well as the aforementioned parameters so that no creeping discharge will occur between the ring-shaped electrode 312 and the tube-line tip member 316.

REFERENCE SIGNS LIST

-   110, 210, 310 . . . Discharging Section -   111, 211 . . . Cylindrical Dielectric Tube -   112, 211 . . . High-Voltage Electrode -   113, 213 . . . Upstream-Side Ground Electrode -   114, 214 . . . Downstream-Side Ground Electrode -   115, 215, 315 . . . High AC Excitation Voltage Power Source -   116, 216, 316 . . . Tube-Line Tip Member -   117, 217, 317 . . . Semiconductor Film -   120, 320 . . . Charge-Collecting Section -   121, 221, 321 . . . Connection Member -   122 . . . Bias Electrode -   123 . . . Collecting Electrode -   126, 326 . . . Sample Introduction Tube -   127, 327 . . . Bias DC Power Source -   128, 328 . . . Current Amplifier -   311 . . . External Dielectric Tube -   312 . . . Ring-Shaped Electrode -   334 . . . Electrode Structure     -   331 . . . Internal Dielectric Tube     -   332 . . . Metallic Tube     -   333 . . . Insulating Tube     -   322 . . . Metallic Wire -   323 . . . Flanged Metallic Tube 

The invention claimed is:
 1. A dielectric barrier discharge ionization detector for ionizing and detecting a sample component in a sample gas by using plasma induced by an electric discharge within a gas passage through which a plasma generation gas containing argon is passed, the detector comprising: a) a dielectric tube made of a dielectric material and containing an upstream section of the gas passage; b) a high-voltage electrode attached to an outer wall of the dielectric tube; c) a ground electrode electrically connected to a ground and arranged so as to face the gas passage, the ground electrode having a surface which faces the gas passage and is covered with a dielectric body, with at least a portion of the surface being located downstream of the high-voltage electrode in a flow direction of the plasma generation gas; d) an AC power source connected to the high-voltage electrode, for applying an AC voltage between the high-voltage electrode and the ground electrode so as to induce a dielectric barrier discharge within the gas passage and thereby generate plasma; and e) a charge-collecting section located downstream of the ground electrode, forming a downstream section of the gas passage, including a sample-gas introducer for introducing a sample gas into the downstream section, and a collecting electrode for collecting ions generated from a sample component in the sample gas by light emitted from the plasma, wherein: a semiconductor film is formed on an inner wall of the dielectric tube over an area where the high-voltage electrode is attached and/or an area located downstream of the aforementioned area and upstream of the charge-collecting section; and when the largest length of the ground electrode which allows a creeping discharge to occur between the high-voltage electrode and the charge-collecting section under the condition that the ground electrode is located between the high-voltage electrode and the charge-collecting section is defined as an initiation distance for a creeping discharge between the high-voltage electrode and the charge-collecting section, a length of the ground electrode in an area located downstream of the high-voltage electrode is longer than the initiation distance for a creeping discharge between the high-voltage electrode and the charge-collecting section.
 2. The dielectric barrier discharge ionization detector according to claim 1, wherein the semiconductor film is a diamond-like-carbon film or titanium dioxide film.
 3. A dielectric barrier discharge ionization detector for ionizing and detecting a sample component in a sample gas by using plasma induced by an electric discharge within a gas passage through which a plasma generation gas containing argon is passed, the detector comprising: a) a dielectric tube made of a dielectric material and containing an upstream section of the gas passage; b) an electrically grounded tube-line tip member made of metal, for introducing the plasma generation gas into the dielectric tube; c) a high-voltage electrode attached to an outer wall of the dielectric tube; d) an electrically grounded upstream-side ground electrode attached to the outer wall of the dielectric tube and located upstream of the high-voltage electrode as well as downstream of the tube-line tip member in a flow direction of the plasma generation gas; e) an electrically grounded downstream-side ground electrode attached to the outer wall of the dielectric tube and located downstream of the high-voltage electrode in the flow direction of the plasma generation gas; f) an AC power source connected to the high-voltage electrode, for applying an AC voltage between the high-voltage electrode and the upstream-side ground electrode as well as between the high-voltage electrode and the downstream-side ground electrode so as to induce a dielectric barrier discharge within the gas passage and thereby generate plasma; and g) a charge-collecting section located downstream of the downstream-side ground electrode, forming a downstream section of the gas passage, including a sample-gas introducer for introducing a sample gas into the downstream section, and a collecting electrode for collecting ions generated from a sample component in the sample gas by light emitted from the plasma, wherein: a semiconductor film is formed on an inner wall of the dielectric tube over an area where the high-voltage electrode is attached and/or an area located downstream of the aforementioned area and upstream of the charge-collecting section; and when the largest length of the downstream-side ground electrode which allows a creeping discharge to occur between the high-voltage electrode and the charge-collecting section under the condition that the downstream-side ground electrode is located between the high-voltage electrode and the charge-collecting section is defined as an initiation distance for a creeping discharge between the high-voltage electrode and the charge-collecting section, a length of the downstream-side ground electrode in the flow direction is longer than the initiation distance for a creeping discharge between the high-voltage electrode and the charge-collecting section.
 4. The dielectric barrier discharge ionization detector according to claim 3, wherein the semiconductor film is a diamond-like-carbon film or titanium dioxide film.
 5. A dielectric barrier discharge ionization detector for ionizing and detecting a sample component in a sample gas by using plasma induced by an electric discharge within a gas passage through which a plasma generation gas containing argon is passed, the detector comprising: a) a dielectric tube made of a dielectric material and containing an upstream section of the gas passage; b) an electrically grounded tube-line tip member made of metal, for introducing the plasma generation gas into the dielectric tube; c) a high-voltage electrode attached to an outer wall of the dielectric tube; d) an electrically grounded upstream-side ground electrode attached to the outer wall of the dielectric tube and located upstream of the high-voltage electrode as well as downstream of the tube-line tip member in a flow direction of the plasma generation gas; e) an electrically grounded downstream-side ground electrode attached to the outer wall of the dielectric tube and located downstream of the high-voltage electrode in the flow direction of the plasma generation gas; f) an AC power source connected to the high-voltage electrode, for applying an AC voltage between the high-voltage electrode and the upstream-side ground electrode as well as between the high-voltage electrode and the downstream-side ground electrode so as to induce a dielectric barrier discharge within the gas passage and thereby generate plasma; and g) a charge-collecting section located downstream of the downstream-side ground electrode, forming a downstream section of the gas passage, including a sample-gas introducer for introducing a sample gas into the downstream section, and a collecting electrode for collecting ions generated from a sample component in the sample gas by light emitted from the plasma, wherein: a semiconductor film is formed on an inner wall of the dielectric tube over an area where the high-voltage electrode is attached and/or an area located upstream of the aforementioned area as well as downstream of the tube-line tip member; and when the largest length of the upstream-side ground electrode which allows a creeping discharge to occur between the high-voltage electrode and the tube-line tip member under the condition that the upstream-side ground electrode is located between the high-voltage electrode and the tube-line tip member is defined as an initiation distance for a creeping discharge between the high-voltage electrode and the tube-line tip member, a length of the upstream-side ground electrode in the flow direction is longer than the initiation distance for a creeping discharge between the high-voltage electrode and the tube-line tip member.
 6. The dielectric barrier discharge ionization detector according to claim 5, wherein the semiconductor film is a diamond-like-carbon film or titanium dioxide film. 